Semiconductor devices are often fabricated through multiple processes, for example, chemical-mechanical planarization (CMP), etching, etc. A CMP process is often used for planarizing surfaces of a wafer with a combination of chemical and mechanical forces. Mechanical grinding alone causes many surface damages, while wet etching alone cannot attain good planarization. The CMP process involves both the mechanical grinding and the wet etching to generate a smooth surface on a wafer, and prepare the wafer for subsequent processes (e.g., photolithography). For example, the CMP process is used to avoid depth focus problems during photolithography.